[v12-engine] Engine braking

Since replacing the distributor cap & plug leads and properly setting idle via
the AAV I have found my UK spec XJ12 now gives noticeable engine braking when I
lift off. I also notice the instant fuel economy reading (what a larf that is on
a 5.3 litre engine!) go to 99. It never did this before - I had no real engine
braking and instant economy only ever got to about 56 mpg. I know there is a
circuit somewhere that detects this condition and shuts off the fuel, but why
has it just started working?

I did also fix the cruise control bellows which required resetting the cable -
could this have changed some setting of the actual throttle cable? Am I just being
paranoid that things just start to work of their own accord (anyone read the book
or see the film Christine)?

1988 XJ12 5.3
1983 XJ6 4.2
1977 Type2 2.0
1965 Mustang 289
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